Because (some) tablets actually are good computers. They achieve extremely high quality (speed, good UX) at the tasks they do by simply not doing most of the tasks a desktop PC does. If you only want to do email and some web browsing, a tablet is actually a higher-quality, faster, easier-to-use machine than a PC.
Because tablets lower the bar for proficiency. If tablets were just lower power, cheaper computers that happened to be portable and have touch screens, you wouldn't see adoption of tablets by casual users.